1. Technical Field
The present disclosure relates generally to microwave antennas. More particularly, the present disclosure relates to thermally tuning coaxial cables for microwave antennas.
2. Background of Related Art
Microwave antennas are used in many applications. For example, medical microwave ablation antennas are used by surgeons. In fact, ablation devices utilizing DC shock, radio frequency (RF) current, ultrasound, microwave, direct heat, or lasers have been introduced and employed to various degrees to ablate biological tissues. Ablation devices may be used in open surgical procedures or are sometimes inserted into catheter devices in order to perform laparoscopic ablation procedures. The catheter incorporating the ablation device is generally inserted into a major vein or artery or through a body cavity. These catheters are then guided to a targeted location in the body (e.g., organ) by manipulating the catheter from the insertion point or the natural body orifice.
During ablation, the dielectric constant of the tissue changes as more water is boiled off and tissue desiccation occurs. The changing value of the dielectric constant alters the antenna's ability to match the originally designed impedance of the antenna. In addition, during microwave ablation in tissue, the impedance of the tissue varies during the course of ablation. This occurrence directly corresponds to how much energy has been deposited into the tissue during the ablation, resulting in temperature increases at the ablation site.
The impedance in the coaxial cable is typically related to the concentricity of the inner conductor in relationship to the outer conductor. In ablation procedures, however, conventional antenna designs only allow for an initial impedance match and as ablation occurs, the increase in mismatch between the tuning point of the antenna and the ablated tissue reduces the efficiency of the energy deposition in the tissue.